The Internet of Things (IoT) is a vision for the future where everything that can benefit from a connection will be connected. Cellular technologies are being developed or evolved to play an indispensable role in the IoT world, particularly machine-type-communication (MTC). MTC is characterized by lower demands on data rates than, for example, mobile broadband, but with higher requirements on low cost device design, better coverage, and the ability to operate for years on batteries without charging or replacing the batteries. To meet the IoT design objectives, 3GPP is currently studying the evolutions of existing 2G/3G/4G Long Term Evolution (LTE) technologies. The current studies under GSM/EDGE Radio Access Network (GERAN) include both Global System for Mobile communications (GSM) evolution and completely new designs.
There are two main so-called “Clean Slate” solutions: (i) Narrowband (NB) Machine-to-Machine (M2M) and (ii) NB Orthogonal Frequency Division Multiple Access (OFDMA). Recently, a merged solution called NB Cellular IoT (CIoT) with NB M2M uplink and NB OFDMA downlink has been proposed and studied in GERAN. These Clean Slate solutions are NB systems with a carrier bandwidth of 200 kHz. The Clean Slate solutions target improved coverage compared to today's GSM systems, long battery life, and low complexity communication design. One intention with the Clean Slate solutions is to deploy them in spectrum that is currently used for GSM, which can be achieved by reducing the bandwidth used by GSM and deploying NB Clean Slate systems in the spectrum that becomes available. Another intention is to reuse existing GSM sites for the deployment of NB Clean Slate systems. 3GPP has decided to move the work on specifying an NB MTC solution from GERAN to RAN.
In existing LTE random access design, random access serves multiple purposes. These purposes include initial access when a user equipment (UE) establishes a radio link, scheduling request, etc. Among others, a main objective of random access is to achieve uplink (UL) synchronization, which is important for maintaining the UL orthogonality in LTE. LTE random access can be either contention-based or contention-free. The contention-based random access procedure consists of four steps:
1) From UE to eNB: Random access preamble;
2) From eNB to UE: Random access response;
3) From UE to eNB: Scheduled transmission; and
4) From eNB to UE: Contention resolution.
Note that only Step 1 involves physical-layer processing specifically designed for random access. The remaining three steps (Steps 2-4) follow the same physical-layer processing used in UL and downlink (DL) data transmission. For contention-free random access, the UE uses reserved preambles assigned by the base station. In this case, contention resolution is not needed, and thus only Steps 1 and 2 are required.
In LTE, random access preambles are sent in the Physical Random Access Channel (PRACH). The PRACH subcarrier spacing is 1.25 kHz, and the preambles are Zadoff-Chu sequences of length 839. A fixed number of preambles (64) are available in each LTE cell. Several preamble formats of different durations of the sequence and cyclic prefix are defined to be used for cells of different sizes. The format configured in a cell is broadcast in the System Information.
One prominent feature of NB LTE is in-band operation (i.e., NB LTE can be deployed within a wideband LTE subcarrier by puncturing one physical resource block (PRB) in the LTE carrier and using it for NB LTE transmission). To enable this in-band operation, it is important to synthesize the NB LTE numerologies with legacy LTE to avoid mutual interference between NB LTE and legacy LTE as much as possible.
In NB LTE, the random access procedure follows its counterpart in LTE. Due to the reduced bandwidth in NB LTE, however, LTE PRACH design cannot be directly applied to NB LTE. As noted above, the LTE PRACH subcarrier spacing is 1.25 kHz and the preambles are Zadoff-Chu sequences of length 839. Thus, the total used bandwidth is 1.0488 MHz (excluding guard band). In contrast, NB LTE is designed to operate with a carrier bandwidth of 200 kHz (more precisely, the usable bandwidth is 180 kHz), making LTE PRACH design inapplicable to NB LTE.
Another relevant consideration is the subcarrier spacing for the Physical Uplink Shared Channel (PUSCH) in NB LTE. In NB LTE, PUSCH may have any suitable subcarrier spacing. As one example, in NB LTE the subcarrier spacing for PUSCH can be 2.5 kHz, which is reduced by 6 times compared to the 15 kHz subcarrier spacing of LTE. One approach to PRACH design for NB LTE would be to reduce the 1.25 kHz subcarrier spacing by 6 times and reuse the length-839 Zadoff-Chu sequences. There are, however, several problems with this design. First, the reduced subcarrier spacing is 208.3 Hz, which is relatively small considering the frequency offset between the device and base station and Doppler shift. Second, the total used bandwidth for PRACH would be 174.8 kHz (208.3*839=174.8 kHz), while the total uplink bandwidth is 180 kHz in NB LTE. As a result, at most two 2.5 kHz subcarriers can be used for PUSCH, and there is no guard band between PUSCH and PRACH when they are frequency multiplexed. As a result, the PUSCH capacity for continuous packet transmissions of users in bad coverage may be limited. Furthermore, different durations of the sequence and cyclic prefix are needed to support cells of different sizes in LTE. This requires more information to be broadcast in System Information. Thus, there is a need for an improved PRACH design for NB LTE.